1. Field of the Invention
The present invention relates to a surface grinding device and to a method for surface grinding a thin-plate workpiece having a sori(warp or bow) or waviness component which is 10 to 30 mm in a period or cycle. More particularly, the present invention is directed to a device and method for surface grinding of a thin-plate workpiece, well suited to surface grinding of a wafer immediately after being sliced by a wire saw or an inner diameter saw (hereinafter referred to also as an "as-cut wafer").
2. Description of the Related Art
In general, a semiconductor wafer is prepared through a slicing step for slicing a silicon ingot that has been pulled by a Czochralski method, into a plurality of thin-plate discs by use of an inner diameter saw or a multi-wire saw, and subjecting the as-cut wafer that has been obtained through the slicing step, to the respective processes of a chamfering step, a lapping step, an etching step, a surface polishing step, and a cleaning step in the stated order.
The as-cut wafer immediately after being sliced has an irregular configuration of plural periods in a bowl-shaped or S-shaped period. In particular, irregularities of 30 mm or more in a bowl-shaped or S-shaped period are called "sori"(warp or bow), and irregularities of 10 to 30 mm in the period are called "waviness". Those irregularities occur in the case where a slicing blade does not go straight due to a slight difference in slicing resistance between the right and left sides of the slicing blade when slicing the wafer, and more particularly they become remarkable in the case where a wire saw is used.
In recent years, a larger diameter a wafer size and a higher flatness level in wafer shape have been strictly demanded with a high integration of the semiconductor devices using such wafers, and it has become difficult for a conventional wafer preparing process to satisfy those demands. In order to meet those demands from customers, surface grinding is very hopeful instead of lapping. In case of adopting this surface grinding processing, a method for preparing a semiconductor wafer may be composed of a slicing step, a chamfering step, a surface grinding step, a polishing step, and a cleaning step, or a slicing step, a chamfering step, a surface grinding step, an etching step, a polishing step, and a cleaning step, etc.
As shown in FIGS. 10 and 11, in general, in a conventional surface grinding device 72, as a system for holding and fixing the wafer, there is employed a system in which one surface of a wafer (W) to be ground is held under vacuum onto a hard chucking plate 76 such as a porous ceramic plate having a large number of through holes 74 via a suction hole 78 that is connected to a vacuum pump, and the other surface thereof is ground by a grinding wheel 80.
However, in the case where the vacuum chucking system in the above-mentioned conventional surface grinding device 72 is applied to a system for supporting an as-cut wafer (W) such as the semiconductor wafer, the waviness on the back surface of the wafer (W) is elastically deformed in conformity with a chucking surface formed in a high flatness due to a vacuum chucking force as shown in FIG. 10.
Upon the surface grinding of the wafer (W), a high flatness level thereof can be obtained since the wafer (W) is vacuum chucked during grinding operation. However, when the vacuum chucking is released after the completion of the grinding operation, an elastically deformed portion of the ground wafer (W) is restored, and a shape of the back surface of the wafer (W) remains in such a manner as transferred to the front surface thereof, thereby making it impossible to obtain a wafer having a predetermined shape.
In particular, a small irregularity having a relatively short period (irregularity of a roughness level) is readily removed even by the conventional surface grinding technique because the shape of the back surface of the wafer (W) is not transferred to the front surface thereof. However, the waviness having a long period of 10 to 30 mm is difficult to remove.
The present inventors made various investigations to solve the problems with the above conventional technique, as a result of which they developed a novel surface grinding method and device, and already proposed the same in Japanese Patent Laid-Open Publication No. 8-66850 and Japanese Patent Application No. 8-80719.
The former is excellent in that the irregularity of the back surface of the wafer can be prevented from being transferred to the front surface thereof by a process in which the back surface of the wafer is held by an adhesive material such as wax in such a manner that it is not elastically deformed, and the front surface of the wafer is then ground. However, since this technique requires steps for coating and removing the wax, there are disadvantages in manufacturing costs and manufacturing time, which should be improved.
The latter is a method of reducing a vacuum chucking pressure by which the wafer is held and fixed to prevent the irregularity of the back surface of the wafer from being transferred to the front surface thereof. Although this technique can remarkably reduce the waviness, there still remains a problem that it can not completely eliminate the waviness.